Stable radical cation salt initiated N-H insertion and related proton-transfer-delay three-component reaction

Article abstract of DOI:10.1016/j.tetlet.2016.04.012

The N-H insertion reaction forms the single addition product exclusively, and does not require slow addition of the diazo component. More importantly, we present the reaction of anilines with both diazoacetates and azodicarboxylates in the presence of catalytic amounts of triarylaminium salt giving the corresponding complex unsymmetrical aminals in high yields. This is the first example of stable radical cation salt promoted proton-transfer-delay three-component reaction.

Full text of DOI:10.1016/j.tetlet.2016.04.012

Tetrahedron Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Stable radical cation salt initiated N–H insertion and related
proton-transfer-delay three-component reaction
⇑
Congde Huo , Haisheng Xie, Caixia Yang, Jie Dong, Yajun Wang
Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou,
Gansu 730070, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The N–H insertion reaction forms the single addition product exclusively, and does not require slow
addition of the diazo component. More importantly, we present the reaction of anilines with both
diazoacetates and azodicarboxylates in the presence of catalytic amounts of triarylaminium salt giving
the corresponding complex unsymmetrical aminals in high yields. This is the first example of stable
radical cation salt promoted proton-transfer-delay three-component reaction.
Received 3 February 2016
Revised 4 April 2016
Accepted 6 April 2016
Available online xxxx
Ó 2016 Elsevier Ltd. All rights reserved.
Keywords:
Radical cation
Triarylaminium salt
N–H insertion
Glycine derivative
Aminal
Carbon–nitrogen (C–N) bond formation is of fundamental inter-
est and has great significance in organic synthesis. The insertion of
proton transfer’ that occurs after the trapping of carbene produces
novel molecules. For example, they reported that diethyl azodicar-
boxylate (DEAD) as a excellent electrophile can be employed to
trap the protic ammonium ylides to obtain unsymmetrical
aminals.6
a-diazo compounds into the N–H bonds of amines is a versatile
and environmentally friendly method of forming C–N bonds.1 This
type of reaction has been used for preparation of many syntheti-
cally and biologically important nitrogen-containing compounds.
For a simple instance, the N–H insertion of anilines with dia-
zoacetates was used in the synthesis of aryl substituted glycine
derivatives, which are the one of the most popular organic
molecules.2 Oxidative dehydrogenative coupling of glycine
derivatives with nucleophiles was reported in large volumes to
One aspect of our research involves the tris(4-bromophenyl)-
aminium hexachloroantimonate (TBPAÅ+SbCl6À) initiated radical
cation mediated transformations.7 This stable triarylaminium salt
is a well-known one electron oxidant and it is commercially avail-
able and relatively inexpensive.8 About 20 years ago, Bauld group
has already shown that triarylaminium salt is an excellent catalyst
for intermolecular cyclopropanation of olefins using ethyl diazoac-
etate (EDA) in dichloromathane.9 In 2011, we discovered that the
reaction of aryl imines with EDA could also be initiated by
TBPAÅ+SbClÀ6 , giving exclusively cis-aziridine carboxylates.7j Given
above results, we hypothesized that TBPAÅ+SbCl6À may also be a
competent initiator for carbene involved N–H insertion reaction.
To the best of our knowledge, the metal-free N–H insertion of
anilines using stable radical cation salt as an initiator has not yet
been reported.
deliver complex
a
-amino acid derivatives.3 Moreover, oxidative
dehydrogenative cyclization of aryl glycine derivatives was also
well developed to form various heterocyclic motifs.4
In recent years, a variety of N–H insertion reactions of anilines
employing transition metal complexes or enzymes as catalysts
have been developed.2 However, there are some drawbacks in
these methods. For example, most catalysts require long reaction
times, high catalyst loadings, and dropwise addition of the diazo
reagent to avoid dimerization byproducts. Therefore, the develop-
ment of N–H insertions of anilines using environmentally benign
metal-free promoter in one-pot process is still of interest.
Here we present the N–H insertion of anilines with diazoac-
etates initiated by TBPAÅ+SbCl6À. More importantly, the reaction of
anilines with both diazoacetates and azodicarboxylates in the pres-
ence of catalytic amounts of TBPAÅ+SbCl6À gives the corresponding
aminals in high yields. In these cases, two C–N bonds were formed
in one reaction. This is the first example of stable radical cation salt
In the past decade, Hu group developed a series of novel multi-
component reactions (MCRs) via trapping of the active onium ylide
intermediates with electrophiles.5,6 In these reactions, a ‘delayed
⇑
Corresponding author.
http://dx.doi.org/10.1016/j.tetlet.2016.04.012
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Huo, C.; et al. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.04.012

2
C. Huo et al. / Tetrahedron Letters xxx (2016) xxx–xxx
Table 2
promoted three-component protic ammonium ylide trapping
reaction.
Triarylaminium salt initiated N–H insertion
Initially, we examined the reaction of p-chloro-aniline with one
equivalent of EDA in the presence of TBPAÅ+SbCl6À (10 mol %) in
dichloromethane (DCM). TBPAÅ+SbClÀ6 induced reactions are typi-
cally carried out in DCM. To our delight, N–H insertion product
3a was isolated in 74% yield within 14 h. A solvent screening
showed that MeNO2 is the best choice. A lower or higher loading
of the initiator resulted in lower yields (Table 1, entries 7 and 8).
No appreciable differences of the transformation were observed
when the reaction was carried out under argon atmosphere. With
the optimized conditions in hand, a range of substituted anilines
were subjected to this radical cation salt promoted N–H function-
alization in the presence of diazoacetates. As summarized in
Table 2, para-, meta-, and ortho-substituted anilines could all be
converted to the designed N–H insertion products in good to high
yields. It’s worth noting that the reactions were run under mild
conditions at ambient temperature in a one-pot fashion, without
the need for slow addition of EDA. No double insertion products
were observed in this N–H insertion reaction.
R3
R3
O
O
H
N
10 mol % initiator a R2
R2
R1
NH2
OR4
4
+
OR4
MeNO3, rt
R1
1
2
N2
a
Entry
R1
R2
R3
R4
4 (yield %)b
1
2
3
4
5
6
7
8
9
10
Cl
H
H
Br
I
NO2
CO2Et
Me
Me
H
Cl
H
H
H
H
H
F
H
H
Cl
H
H
H
H
H
H
H
Et
Et
Et
Et
Et
Et
Et
Et
Et
tBu
4a (84)
4b (79)
4c (86)
4d (95)
4e (67)
4g (57)
4h (65)
4i (62)
4j (89)
4p (66)
Cl
H
Cl
Reaction conditions: 1 (1.0 mmol), 2 (1.0 mmol), TBPAÅ+SbCl6À (10 mol %), sol-
vent (2.0 mL), 14–25 h.
a
b
Isolated yields of the isolated products.
The success of N–H insertion reaction of anilines using catalytic
amounts of triarylaminium salt encouraged us to develop a more
general and useful method. We subsequently turned our attention
to investigate the three-component proton-transfer-delay reaction
of anilines, diazoacetates, and azodicarboxylates to construct com-
plex aminals under the initiation of TBPAÅ+SbCl6À. These aminals are
protocol, a range of substituted anilines were subjected to this rad-
ical cation salt promoted multi-component reaction in the pres-
ence of EDA and DEAD. In terms of substituents on the aniline
ring, electron-donating and electron-withdrawing groups were
all well tolerated. As summarized in Scheme 1, all of the aniline
derivatives, including para-, meta-, ortho-, and poly-substituted
anilines, could be converted to the designed aminal products in
good to high yields (5a–5m). The NMR spectra of these aminal
compounds are not very clear.11 So we also confirmed the struc-
tures of 5m by the X-ray diffraction.12 The different diazoacetates
or azodicarboxylates also provided the corresponding animal prod-
ucts (5n, 5o, 5p) in good yields.
To explore the reaction mechanism, we carried out the follow-
ing control experiments. 1g itself shown no reaction under stan-
dard reaction conditions. (Table 3, entry 1). 2a itself decomposed
completely under standard reaction conditions. At the same time,
decolorization of the triarylaminium salt occurred rapidly and N2
gas evolution accompanied. (Table 3, entry 2). The reaction of 1g
with 2a in the absence of TBPAÅ+SbClÀ6 afforded no product (Table 3,
entry 3). The reaction of 1g with 2a under standard reaction condi-
tions afforded designed N–H insertion product in good yields
(Table 3, entry 4). The reaction of 1g with 2a and 3a in the absence
of TBPAÅ+SbClÀ6 afforded no product (Table 3, entry 5). The reaction
of 1g with 2a and 3a under standard reaction conditions afforded
designed proton-transfer-delay three-component reaction product
in high yields (Table 3, entry 6). These experiments suggest that
TBPAÅ+SbClÀ6 is the crucial carbene initiator to this transformation.
In order to avoid the influence of the trace amount of SbCl5 or any
other possibly existing trace amounts of Lewis acids or Bronsted
acids in the reaction mixture, an equimolar amount of hindered
nonnucleophilic base 2,6-di-tert-butylpyridine (DBP) was added
as an acid scavenger.13 No obvious inhibition was observed and
the reaction performed as effectively as before (Table 3, entry 7).
The possibility of an acid-promoted mechanism is ruled out. It is
noteworthy to mention that no reaction occurred between glycine
ester 4g and DEAD (3a) under standard reaction conditions
(Table 3, entry 8). This might because the electrophilic attack of
DEAD is faster than [1,2]-hydrogen shift. These experiments pro-
vide experimental evidence for the existence of the protic onium
ylide intermediates.
useful building blocks for the synthesis of complex
a-hydrazino
acid frameworks which are usually used for chemical modification
of peptide backbones.10,11 Multi-component reactions (MCRs) have
already become a powerful synthetic tool in organic chemistry
now, and they allow the construction of complex molecules from
simple and easily available substrates with high efficiency and
step-economic feature. To our delight, after slight tuning of our
reaction conditions (decreasing the initiator loading from 10 mol
% to 5 mol %), reaction of p-chloro-aniline with EDA and DEAD fur-
nished the desired product 5a with a complete conversion and an
excellent isolated yield. The standard reaction condition was then
established. To a stirred mixture of anilines (1, 1.0 mmol), diazoac-
etates (2, 1.0 mmol) and azodicarboxylates (3, 1.0 mmol) in MeNO2
(2.0 mL) under argon atmosphere, TBPAÅ+SbCl6À (5 mol %) were
added. The reactions were performed at room temperature and
completed within 12–26 h as monitored by TLC. The products 5
were isolated by column chromatographic separation. In order to
investigate the substrate scope of this TBPAÅ+SbCl6À initiation
Table 1
Screening of reaction conditions
O
Cl
a
Cl
TBPA SbCl6
Solvent
O
+
O
NH2
1a
N
H
N2
O
2a
3aa
a
Entry
Loading (mol %)
Solvent
Atmosphere
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
10
10
10
10
10
5
CH2Cl2
CH3CN
C2H4Cl2
Toluene
THF
CH3NO2
CH3NO2
CH3NO2
CH3NO2
Air
Air
Air
Air
Air
Air
Air
Air
Ar
74
70
67
52
55
84
62
83
86
15
10
Although the mechanistic details of this transformation are not
clear at this stage, on the basis of the above results, a plausible
mechanism for this triarylaminium salt initiated transformation
is illustrated in Scheme 2. EDA was activated by triarylaminium
a
Reaction conditions: 1a (1.0 mmol), 2a (1.0 mmol), TBPAÅ+SbCl6À (10 mol %),
solvent (2.0 mL), 14 h, rt.
b
Isolated yields of the isolated products.
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C. Huo et al. / Tetrahedron Letters xxx (2016) xxx–xxx
3
O
NH2
R4
O
H
O
Ar
N
R4
R5
a
2
O
O
1
N2
5 mol % initiator
MeNO3, rt
+
N
O
HN
O
R5
N
O
O
R5
O
O
N
R5
5
5m
3
O
O
O
O
H
Cl
O
O
H
N
H
N
H
N
Cl
N
O
O
O
O
O
O
O
O
N
O
N
O
N
O
N
O
Cl
HN
HN
HN
Br
HN
O
O
O
O
O
O
O
5a (86%)b
5c (40%)
5b (47%)
5d (75%)
O
O
O
O
H
N
H
N
H
N
H
N
O
O
O
O
O
O
O
O
N
O
N
N
O
O
N
O
F
HN
I
HN
O2N
HN
HN
O
O
O
O
O
O
O
O
O
O
5e (79%)
5f (83%)
5g (72%)
5h (51%)
O
O
O
O
O
H
N
H
N
H
N
H
N
F
Cl
O
O
O
O
O
O
N
O
N
O
N
O
N
O
HN
HN
HN
O
HN
O
O
O
O
O
O
O
O
O
5i (76%)
5j (77%)
5l (81%)
5k (91%)
O
O
O
O
O
H
N
H
N
H
N
H
N
O
O
O
O
O
O
N
O
N
O
N
O
O
N
O
Cl
HN
HN
Cl
HN
Cl
HN
O
O
O
O
O
O
O
O
5o (56%)
5n (88%)
5m (90%)
5p (40%)
Scheme 1. Triarylaminium salt initiated proton-transfer-delay three-component reaction and X-ray-derived ORTEP drawing of compound 5 m. Reaction conditions:
(a) 1 (1.0 mmol), 2 (1.0 mmol), 3 (1.0 mmol), TBPAÅ+SbCl6À (5 mol %), solvent (2.0 mL), 12–26 h. (b) Isolated yields of the isolated products.
Table 3
Control experiments
Entry
1
Substrates
Conditions
Products
10 mol % TBPA+ÅSbCl6À
MeNO3, rt
NH2
—
—
—
No reaction
O2N
1g
10 mol % TBPA+ÅSbCl6À
MeNO3, rt
O
2
—
Decomposed + N2
O
N2
2a
3
4
1g
1g
2a
2a
—
—
MeNO3, rt
No reaction
4g (57%)
10 mol % TBPA+ÅSbCl6À
MeNO3, rt
O
N
O
5
1g
2a
MeNO3, rt
No reaction
O
N
3a
O
6
7
1g
1g
2a
2a
3a
3a
5 mol % TBPA+ÅSbCl6À
MeNO3, rt
5g (72%)
4g (0%)
5g (68%)
4g (0%)
5 mol % TBPA+ÅSbCl6À
5 mol % DBP
MeNO3, rt
5 mol % TBPA+ÅSbCl6À
MeNO3, rt
O
H
N
O
8
3a
No reaction
O2N
4g
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4
C. Huo et al. / Tetrahedron Letters xxx (2016) xxx–xxx
O
O
O
H H
N
H
N
R4
R4
NH2
R4
proton transfer
O
O
O
Ar
Ar
Ar
4
B
A
1
O
Initiated
by stable radical salt
R5
N
O
O
N
R5
O
3 (electrophile)
O
R4
O
2
O
H
O
O
H
H
N
N2
R4
R5
N
R4
O
delayed
proton transfer
O
O
Ar
Ar
N
O
O
N
O
N
R5
HN
O
O
O
R5
R5
C
5
Scheme 2. Proposed mechanism of 1a and 2.
salt TBPAÅ+SbClÀ6 to give the carbene intermediate A. Intermediate
A was then attacked by anilines 1 to form the protic ammonium
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a [1,2]-hydrogen shift
occurred to give the N–H insertion product 4. In three-component
reaction, nucleophilic addition of protic ammonium ylide B to
azodicarboxylates (an excellent electrophile) occurred first to gen-
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gave products 5.
In conclusion, an efficient stable radical cation salt TBPAÅ+SbCl6À
initiated N–H insertion reaction has been developed. Furthermore,
we have discovered a novel catalytic amounts of TBPAÅ+SbCl6À pro-
moted three-component reaction for the preparation of complex
aminals through an protic ammonium ylide trapping protocol.
Currently, we are investigating other triarylaminium salt promoted
proton-transfer-delay multi-component reactions and their further
applications.
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We thank the National Natural Science Foundation of China
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2016.04.
012.
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Please cite this article in press as: Huo, C.; et al. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.04.012